SDH and SONET are both examples of protocols suitable for use in sending data over synchronous transmission networks. Such networks comprise a plurality of network elements (also referred to as nodes) connected together by optical fibre or any other suitable type of connection. Each node comprises a synchronous transmission apparatus, or network equipment, in the form of, for example, a multiplexer, a cross-connect, a regenerator, or other signal processing or switching apparatus. The synchronous transmission apparatus, or network elements, incorporated in the network are typically synchronised (locked in frequency but not in phase) to an overall network clock.
The SONET (Synchronous Optical Network) Standard defined by ANSI (American National Standards Institute) is similar in many respects to the SDH (Synchronous Digital Hierarchy) standard defined by the ITU-T (International Telecommunications Union Telecommunications Standardization Sector).
In particular situations it is required to send a plurality of streams of data in parallel between two network nodes in a synchronous transmission network and also to maintain a particular alignment between those streams of data. For example, a data bus which generates parallel streams of data that must be transported in such a manner that the differential delay between the streams is kept to that which can be tolerated by the system. Otherwise the receiver will be unable to recover the transmitted signal, and the link will either fail, or be forced to operate at a lower bandwidth.
The problem of how to maintain alignment between a plurality of streams of data in this way is exacerbated when the streams of data must be transmitted over long distances such as in a Wide Area Network (WAN). This is because any differential delay experienced will tend to be larger for longer distances. Delays occur due to transmission delays along fiber links and delays within the nodes themselves. Protection switching may also occur in the network, to causing the delay experienced by some, or all, of the streams to change over time.
A typical delay for a STM-1 frame at a node is 90 bytes per VC-4. This gives a lowest time delay per node of the order of 5 μs. Additionally, the delay incurred due to the transmission along optical fiber is of the order of 5 μs per km. Thus, if 2 streams of data are sent across a network by different routes, having a round trip geographical distance difference of 1000 km, the streams could arrive at the same destination 5 ms apart due just to the difference in fiber delay between the two routes. A differential delay between source and destination over a large network of the order of 10 ms may be incurred.
Previously this problem has been addressed by locating the equipment sufficiently close together, such that the differential delays would not exceed that allowed by the equipment, or by employing dedicated ‘dark fibre’, and ensuring that the lengths of the individual fibres was closely matched.
An alternative approach would be to multiplex all the streams of data into a single serial stream which is demultiplexed when it reaches its destination. However, this approach requires a transport path which operates at a higher bit rate than the sum of the streams to be transported, and that when protection switching occurs, all these paths will be switched simultaneously. These conditions will not always be achievable in a given synchronous network.